1. Field of Invention
The present invention relates to bullets for obtaining cores or samples from formations adjacent to well bores.
2. Description of Prior Art
In petroleum exploration, it is desirable to determine the nature and composition of rock formations at various depths in a well bore. On way of doing so has been the use of sampling bullets. Typically, a number of such bullets are mounted with associated explosive charges on a sampling tool for movement through the well bore.
At a selected depth, the charge for one or more of the bullets is ignited, firing the bullet into the rock formation. The bullet is provided with a center opening or passage which gathers a core or sample of the formation rock as the bullet enters. The bullet enters the rock a distance of about one inch but is kept connected to the tool by wire cables. When the tool is moved away from the formation, the bullet is extracted from the formation and hangs from the tool. After all bullets have been fired, the cores in them may be transported by the tool to the surface for analysis.
Applicant has been making and supplying bullets for a number of years for a major oil well service company. Although satisfactory for most purposes, these types of bullets had a relatively short service life due to severe service conditions present when the bullet was fired into the rock formations.
After ten or so firings, the bullets were likely to break or fracture on any future shot. If the bullet broke in service, either no core was obtained or the core sample fell loose during movement of the sampling tool to the surface. Further, bullet failure was undetected until the sampling tool was pulled to the surface, so that additional runs of the sampling tool into the well bore were required. Further, even where the bullet satisfactorily entered the formation and did not fracture or break, the core would work loose from the bullet during tool movement of the sampling tool in the well bore so that the core could not be recovered.